Engineering 3D-Printed UHPC: Optimising Fibre Content and Printing Direction for Enhanced Mechanical Performance

3D-printed ultra-high-performance concrete (3DP-UHPC), which combines high strength, high toughness, and construction flexibility, provides a solution to the reinforcement difficulty of conventional printed concrete. However, the layer-by-layer printing process induces pronounced anisotropy in compressive and flexural properties, which remains a key challenge for engineering applications. We used X-ray computed tomography (X-CT) to quantitatively analyse the internal pores and to reveal the orientation and distribution of steel fibres in 3DP-UHPC. Splitting tensile tests showed that the interlayer splitting tensile strength is approximately 49.3% lower than the conventional splitting tensile strength, and increasing fibre content has a limited strengthening effect. Analysis of the compressive anisotropy index \( \lambda_\text{c} \) indicates that, at \( V_\text{f} = 0% \), anisotropy is mainly influenced by weak interlayer interfaces and pore distribution. Linear fitting shows that a small fibre addition (\( V_\text{f} \leq 1.28% \)) reduces compressive anisotropy, whereas higher \( V_\text{f} \) leads to an increase in \( \lambda_\text{c} \) due to fibre orientation. For the flexural anisotropy index \( \lambda_\text{f} \), interstrip interfaces and pores dominate at \( V_\text{f} = 0% \) and \( 0.5% \), while fibre distribution governs \( \lambda_\text{f} \) when \( V_\text{f} \geq 1.0% \), with an increase exceeding 60%. Within \( V_\text{f} = 0%-2.0% \), \( \lambda_\text{c} \) remains higher than \( \lambda_\text{f} \), indicating relatively stable compressive performance. These results provide experimental evidence for optimising the compressive and flexural performance of 3DP-UHPC components in engineering applications.